Our ability to see spatial patterns depends, to a great extent, on the prevailing illumination. The long term goals of this study are twofold: to account for the changes in spatial thresholds with light level, and to characterize the perception of suprathreshold patterns under dim illumination. Experiments are proposed to differentiate optical from neural constraints and to estimate the limits imposed on spatial vision by neural sampling, filtering and noise at various light levels. The specific aims of this study are: 1) to quantify the effect of optical defocus in the loss of spatial vision at dim light levels, 2) to examine how grating visibility is influenced by the adaptation state of the surrounding retina, 3) to differentiate neural spatial filtering and sampling constraints under dim illumination, 4) to investigate the spatial sampling properties of the visual system at various light levels, and 5) to characterize the appearance of suprathreshold moving and stationary patterns under dim illumination. Measurements will be made on adult human observers, using stimuli and objective psychophysical techniques that optimize visual performance. This study will provide a comprehensive set of acuity and contrast sensitivity data for normal observers that will be useful in modeling and clinical applications.